1. Field of the Invention
This invention relates to quality assurance (QA) methods for surface treatment of a substrate surface such as peening and, more particularly, for using holographic interferometry for quality assurance of laser shock peening processes.
2. Description of Related Art
Laser shock peening or laser shock processing, as it is also referred to, is a process for producing a region of deep compressive residual stresses imparted by laser shock peening a surface area of a workpiece. Laser shock peening typically uses multiple radiation pulses from high power pulsed lasers to produce shock waves on the surface of a workpiece similar to methods disclosed in U.S. Pat. No. 3,850,698, entitled "Altering Material Properties"; U.S. Pat. No. 4,401,477, entitled "Laser Shock Processing"; and U.S. Pat. No. 5,131,957, entitled "Material Properties". Laser shock peening, as understood in the art and as used herein, means utilizing a laser beam from a laser beam source to produce a strong localized compressive force on a portion of a surface by producing an explosive force by instantaneous ablation or vaporization of a painted or coated or uncoated surface. Laser peening has been utilized to create a compressively stressed protection layer at the outer surface of a workpiece which is known to considerably increase the resistance of the workpiece to fatigue failure as disclosed in U.S. Pat. No. 4,937,421, entitled "Laser Peening System and Method". These methods typically employ a curtain of water flowed over the workpiece or some other method to provide a confining medium to confine and redirect the process generated shock waves into the bulk of the material of a component being LSP'D to create the beneficial compressive residual stresses.
Laser shock peening is being developed for many applications in the gas turbine engine field, some of which are disclosed in the following co-pending U.S. patent application Ser. No. 08/362,362 entitled "ON THE FLY LASER SHOCK PEENING", filed Dec. 22, 1994; and U.S. Pat. No. 5,591,009, entitled "Laser shock peened gas turbine engine fan blade edges"; U.S. Pat. No. 5,569,018, entitled "Technique to prevent or divert cracks"; U.S. Pat. No. 5,531,570, entitled "Distortion control for laser shock peened gas turbine engine compressor blade edges"; U.S. Pat. No. 5,492,447, entitled "Laser shock peened rotor components for turbomachinery"; U.S. Pat. No. 5,674,329, entitled "Adhesive tape covered laser shock peening"; and U.S. Pat. No. 5,674,328, entitled "Dry tape covered laser shock peening", all of which are assigned to the present Assignee. These applications, as well as others, are in need of efficient quality assurance testing during production runs using laser shock peening.
Laser shock peening processes have been developed to simultaneously LSP pressure and suction sides of leading and trailing edges of fan and compressor airfoils and blades as disclosed in U.S. Pat. No. 5,591,009 entitled "Laser shock peened gas turbine engine fan blade edges" and U.S. Pat. No. 5,531,570 entitled "Distortion control for laser shock peened gas turbine engine compressor blade edges". LSP is a deep treatment of the material and it is desirable to have a quality assurance test that is indicative of a volumetric LSP effect. It is also desirable to have a QA method that is compatible with a dual sided or simultaneous dual sided LSP process wherein substantially equal compressive residual stresses are imparted to both sides of a workpiece, i.e. along the leading edge of a gas turbine engine fan blade.
Conventional high cycle fatigue (HCF) testing of blades having leading edges which are LSP'd and notched before testing has been tried as a quality assurance technique. This method is destructive of the workpiece, fairly expensive and time consuming to carry out, and significantly slows production and the process of qualifying LSP'd components. An improved quality assurance method of measurement and control of LSP that is a non-destructive evaluation (NDE), inexpensive, accurate, and quick is highly desirable. It is also desirable to have an NDE quality assurance method that is relatively inexpensive and sufficiently economical to be used on each workpiece instead of a sampling of workpieces. LSP is a process that, as any production technique, involves machinery and is time consuming and expensive. Therefore, any techniques that can reduce the amount or complexity of production machinery and/or production time are highly desirable.
The present invention uses holographic interferometry for laser shock peening quality assurance. Holographic interferometry is a well known non-destructive evaluation or examination (NDE) technology used to test and evaluate surfaces. Interferometry has been known and studied for many years. In its simplest form optical interferometry involves providing a measuring beam and a reference beam so that the measuring beam is reflected off of the test item and the reference beam is reflected off of a fixed object, typically a mirror, the two beams are recombined, and an interference or fringe pattern is created which is proportional to the phase difference imparted on the measuring beam by the surface of the test object. The present invention provides a holographic interferometry method to evaluate a region of deep compressive residual stresses imparted by laser shock peening a surface area of a workpiece.
U.S. Pat. No. 5,523,839 discloses an on-line manufacturing process control system and method that uses a twin or dual beam heterodyned optical interferometer to monitor changes in the surface characteristics of an object while the object is being manufactured. Changes in surface characteristics can then be used as data by automated equipment to determine whether the manufactured object monitored at a given time meets predetermined quality standards.
U.S. Pat. No. 4,725,142 discloses a method and apparatus for holographic interferometry inspection to determine and characterize effects of intermittent stressing of objects.
U.S. Pat. No. 4,139,302 discloses a method and apparatus for interferometric deformation analysis that produces superimposed interference fringes arrayed as a function of the deformation which results in the object from an applied stress, which may be mechanical, thermal, or the like. This method is used to analyze anomalous deformation which may have resulted from the design of the object or from anomalous structural characteristics of the object, such as cracks, subsurface separations, voids or inclusions or areas of non-uniform strength which affect the surface deformation. The U.S. Pat. No. 4,139,302 patent offers as an alternative to holographic interferometry, its method and suggests that its invention may be employed in all the applications that have been suggested for holographic interferometry including the detection of cracks in welds, of subsurface voids, separations and non-uniformities in vehicle tires, separations in sandwich honeycombs and other fabricated structures, etc. The object is first illuminated with coherent light. The illuminated surface is then photographed with a camera having an optical wedge disposed over half of its lens to record two slightly displaced overlapping images of the object on the camera film. The object is then stressed by changing the ambient temperature or pressure or other mechanical loading, and the undeveloped film is exposed to a second set of overlapping images. The developed photograph contains a set of equal amplitude fringes representing the interference pattern between the two fringe sets generated by the two exposures and arrayed as a function of the strain in the object as a result of the stress. To render the fringe set visible, an image of the object is projected on a screen through a Fourier, fringe-frequency sensitive filter, which enhances the fringe contrast on the resulting image of the object.
U.S. Pat. No. 5,432,595 discloses a method and apparatus for measuring residual stress in a material comprising the steps of establishing a speckle pattern on the surface with a first laser then heating a portion of that pattern with an infrared laser until the surface plastically deforms. Then comparing the speckle patterns before and after deformation by subtracting one pattern from the other will produce a fringe pattern that serves as a visual and quantitative indication of the degree to which the plasticized surface responded to the stress during heating and enables calculation of the stress. The method was developed for accurately measuring the residual stresses in individual parts so that a designer can predict failure with greater certainty and design to avoid failure or to relieve the stresses. The method is designed to measure surface stresses at the surface of a metal object.
However, none of the prior art uses holographic interferometry to analyze or inspect deep compressive residual stresses imparted by laser shock peening a surface area of a workpiece for the purpose of quality assurance. The prior art does not appear to offer or suggest any use of holographic interferometry or any type of interferometry as a method for qualitative assurance of laser shock peening.